In rotary machines, heat is generated from rotating components, such as bearings and gears, as a result of friction, windage and oil churning etc. Particularly within combustion engines an oil system is intended to deliver oil at an adequate flow rate and temperature to lubricate and cool components. The heated oil is collected and cooled via a suitable cooling system so as to establish a continuous heating and cooling cycle to maintain oil temperature within desired temperature limits. Such an oil system typically forms part of a larger engine heat management system.
A fuel/oil heat exchanger may be used to cool hot oil, whereby the engine fuel provides a primary coolant. Additional cooling may be provided via an air/oil heat exchanger if the fuel heat dissipation capacity is insufficient to adequately maintain the desired oil and fuel operating temperatures.
It is known to use air taken from a low pressure compressor stage as a source of cooling air, whereby air at a relatively low temperature is driven through the heat exchanger. However the pressure rise imparted by a low pressure compressor limits the cooling capacity of such an arrangement. Also there is uncertainty or inconsistency of the air supply at lower engine power/speed conditions. Whilst downstream compression stages of a compressor arrangement can provide higher pressure air, the work done by the compressor increases the air temperature at those stages, such that it becomes less suitable or else entirely unusable as a coolant supply. Accordingly the choice of a suitable compressor stage as a coolant supply is limited, as is the cooling capacity of air supplied thereby.
Also, some bleed air is typically taken from the compressor stages as part of the air system and fed into bearing chambers of a gas turbine engine in order to ensure correct bearing chamber sealing. The hotter compressor air causes an increased oil temperature, thereby increasing the demands on oil heat exchangers.
An alternative cooling air source for aircraft gas turbine engines is ram air due to the forward motion of the aircraft/engine. Ambient air in a ram air intake, as opposed to air on which work has been done by a compressor, can provide a cooler air source such a comparative cooling capacity to that provided by air from a compressor stage can be achieved by ram air that a lower coolant flow rate. However ram air intake is subject to the relative movement in a direction of travel between the engine and ambient air. Accordingly, particularly at low levels of operation (e.g. at low engine/aircraft speeds), the ram air pressure is typically insufficient to drive an adequate flow of cooling air through the relevant portions of the cooling system.
It is known to provide an additional cooling arrangement in conjunction with a ram air intake, such as a so-called ejector system, in which high pressure air is taken from a compressor stage to drive coolant through the system. However such arrangements represent a loss of efficiency for the engine as a whole. The weight and cost associated with the additional pipework and consumption of high pressure air as ejector driving flow are specific points of concern.
The cooling air needed for the whole engine oil and heat management system of a Gas Turbine Engine (GTE) could be as high as 10% or more of the total core engine airflow. In light of the ongoing trend towards improved gas turbine engine efficiency, particularly in the aerospace industry, such a coolant flow is considered to have a significant impact on the engine performance.
It is an aim of the present invention to provide an improved coolant system which can solve or at least mitigate problems associated with the prior art, for example by offering greater cooling efficiency and/or reduced complexity.